Light emitting device and method of manufacturing a light emitting device

ABSTRACT

A light emitting device comprising a heat sink, a dielectric layer arranged on the heat sink, a heat conductive layer arranged on the dielectric layer, an undercoating arranged on at least a part of the heat conductive layer, and a light emitting chip attached to the heat conductive layer by means of the undercoating.

RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 61/132,888 which was filed on Jun. 24, 2008, thedisclosure content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to a light emitting device comprising a lightemitting chip attached on a heat conductive layer by means of anundercoating.

BACKGROUND OF THE INVENTION

One problem of the existing light emitting devices is the high thermalresistance from the device to the ambient air. This is due to thestack-up of the device like, for example, the use of a printed circuitboard or a lead frame.

Since heat conduction of the devices is directly related to efficiencyand life time of the light emitting devices, it is desirable to producedevices with good heat conduction.

In addition, the complexity of light emitting devices comprising aprinted circuit board is very high so reducing this is an additionalgoal of some embodiments of this invention.

Most of the light emitting devices comprise an LED chip which isattached to a leadframe. The leadframe is usually metal and is encasedin a plastic or ceramic package. This package is normally made of aplastic or ceramic material. These materials have a low thermalconductivity. In addition, the packaged chip can be attached to theboard by a glue or solder and the board is attached to the heat sink bya glue, thermal interface like a paste, grease or an epoxid or anotheradhesive. This glue and the adhesive materials also have a low thermalconductivity. Furthermore, the material of the printed circuit boardwhich is typically an FR4-material (woven glass and epoxy) or aCEM-material (cotton paper and epoxy or woven glass and epoxy), bothhave a low thermal conductivity.

The heat which is produced during operating the light emitting device bythe chip has to pass the package into the board substrate and thethermal interface material or air between the board and the heat sinkuntil it reaches the heat sink and can be dissipated to the ambient.

SUMMARY OF THE INVENTION

One embodiment of the invention is directed to a light emitting devicecomprising a heat sink, a dielectric layer arranged on the heat sink, aheat conductive layer arranged on the dielectric layer, an undercoatingarranged on at least a part of the heat conductive layer, and a lightemitting chip attached on the heat conductive layer by means of theundercoating. The undercoating can be also electrically conductive andpart of a circuit.

This light emitting device has a better heat conduction and thermalresistance than a light emitting device comprising a leadframe or aprinted circuit board. Here, the light emitting chip is directlyattached to a heat conductive layer by means of an undercoating. So theenergy which is produced during operating the light emitting device caneasily be transferred from the chip to the heat conductive layer via thethermally conductive undercoating. Because of the good thermalconductivity of the heat conductive layer, the heat is quicklytransferred away from the chip to the dielectric layer and from there tothe heat sink. Due to the efficient heat transfer of the device, thelight efficiency and the life time of the device is increased.Additionally, the complexity of some embodiments of the device isreduced, because these devices do not comprise a leadframe or a printedcircuit board.

In another embodiment the heat conductive layer comprises a contactpoint in a distance to the undercoating.

The surface of the contact point can be enlarged compared to the surfaceof the heat conductive layer surrounding the contact point so thatenergy which is applied to the contact point is absorbed moreefficiently and faster than if the energy would be applied directly tothe surface of the heat conductive layer. Furthermore it is possiblethat the contact point comprises an absorption material, which has abetter light absorption than the heat conductive layer.

This contact point makes it possible to produce a light emitting devicein a new way. This contact point is formed in a way that it can absorbthermal energy more easily and faster than the surface of the heatconducting layer. If in a production step heat is applied to thiscontact point the thermal energy is quickly absorbed. The increasedabsorbance of the energy at the contact points can be based on thegeometric form of the contact points or in the material the contactpoint is made of.

In another embodiment, the thermal conductivity of the heat conductivelayer is greater than 350 W/m*K.

Because of the good thermal conductivity of the heat conductive layer,the heat which is produced by the chip during operation can be quicklytransported away from the chip into the direction of the heat sink.Further more because of the high heat conductivity of the heatconductive layer the heat, which is transferred from the chip to theheat conductive layer is quickly distributed within the whole heatconductive layer thereby avoiding an accumulation of the heat in thearea where the heat conductive layer is in contact with theundercoating. Therefore the thermal conductivity of the light emittingdevice according to some embodiments of the invention is improvedcompared to conventional devices. Additionally, the high conductivity ofthe heat conductive layer makes it possible to produce the device in anew way. Energy which is applied to the conductive layer, for example tothe contact points or to other points in a distance of the undercoating,can be quickly transferred from the contact point where the energy isapplied to the undercoating. The heat conductive layer could comprisecopper for example.

In another embodiment, the thermal conductivity of the heat conductivelayer is greater than the thermal conductivity of the dielectric layer.

Because of the greater thermal conductivity of the heat conductivelayer, the heat which is applied to the conductive layer is transportedwithin the heat conductive layer towards the undercoating in a quickerway than transported from the heat conductive layer through thedielectric layer to the heat sink. Due to that the whole heat conductivelayer is heated before the energy is transferred through the dielectriclayer. The dielectric layer could be made of an inorganic material likea oxide or of a polymeric organic material like a plastic. This makes itpossible to get a uniform or nearly uniform dispersion of the thermalenergy in the heat conductive layer.

In another embodiment, the contact point has a geometric form thatincreases the absorption of the thermal energy.

The geometric form can be, for example, tines, waves or groovings. Thesurfaces of the contact points can be enlarged compared to the surfaceof the heat conductive layer surrounding the contact point so thatenergy which is applied to the contact point is absorbed moreefficiently and faster than if the energy would be applied directly tothe surface of the heat conductive layer.

In another embodiment, the contact point comprises an absorptionmaterial that increases the absorption of light.

If the energy is applied to the contact point, for example by a laser,the absorption material of the contact point could be matched to thewave length of the laser light. Thereby, the absorption of the energy atthe contact point is much better than the absorption of the othersurface of the heat conductive layer surrounding the contact point.Hereby, the absorption material can be located on the top of the contactpoints or distributed in the whole contact point area. This absorptionmaterial makes it possible to heat up the heat conductive layerefficiently by a laser system. For the absorption material for examplecarbon black could be used.

In another embodiment, the contact point and the heat conductive layercontain the same heat conductive material.

It is possible to form a contact point directly in the heat conductivelayer. For example it is possible to only enlarge the surface of theheat conductive layer in the area of the contact point. Furthermore, itis possible to apply an absorption material onto the surface of the heatconductive layer in the area of the contact point. It should be ensuredthat also the contact point has a high heat conductivity, that theenergy which is applied to the contact point is quickly transferred fromthe contact point to the heat conductive layer.

In another embodiment, the light emitting device comprises a pluralityof contact points.

A “plurality” means in this case more than one contact point. Thesecontact points are preferably arranged symmetrically around theundercoating for example in a circular or rectangular arrangement.Symmetrically means in this case that the contact points are arranged inthe same distance to the undercoating, each on an opposite side of theundercoating. This makes it possible to heat up the heat conductivelayer uniformly by simultaneously applying heat to the contact points,which are located on opposite sides of the undercoating.

In another embodiment, the undercoating comprises a solder material.

The solder material can be used to attach the chip to the heatconductive layer. Also, the solder material can have a good heatconductivity for transporting the heat from the chip to the heatconductive layer. Furthermore, the solder material can serve as anelectric contact between the chip and the heat conductive layer. Thesolder material could be softened by a reflow soldering process. Assolder material a lead free solder can be used comprising an alloy oftin, silver and zinc.

In another embodiment, the melting point of the undercoating is lowerthan the melting point of the heat conductive layer.

It is possible to soften the undercoating without applying energydirectly to the undercoating. Energy can be applied to the heatconductive layer, for example via the contact points, and then could betransferred from the contact points to the heat conductive layer andfurther to the undercoating. Thereby, the undercoating could be softenedby the heat, without softening the heat conductive layer itself.

Apart from the light emitting device, a method of manufacturing a lightemitting device is disclosed.

A variant of the method of manufacturing a light emitting devicecomprises the method steps of providing a heat sink in a method step A),applying a dielectric layer to the heat sink, as step B), arranging aheat conductive layer on the dielectric layer, as step C), applying anundercoating to a part of the heat conductive layer, as step D),positioning a chip on the undercoating, as step E), applying energy tothe heat conductive layer at a distance to the chip, whereby the heatconductive layer is heated and the heat is transported through the heatconductive layer to the undercoating, such that the undercoating issoftened, as step F) and in a step G), hardening the undercoating,thereby attaching the chip to the heat conductive layer.

During this manufacturing method, the undercoating is softened withoutapplying energy directly to the undercoating. The energy is applied tothe heat conductive layer and than transported in the heat conductivelayer to the undercoating. If the energy is applied in a distance to theundercoating and therefore in a distance to the chip there is no dangerthat the chip could be damaged by the energy. Even by applying a highenergy, the energy is distributed through the whole heat conductivelayer so it is more uniform and curbed when it reaches the undercoating.So in this method, the undercoating is softened in a smooth way. For theundercoating, a material could be used which comprises a soldermaterial. The chip is mechanically connected to the heat conductivelayer via the undercoating in an electro conductive and thermoconductive way. Because of the lower heat conductivity of the dielectriclayer, the heat which is applied to the heat conductive layer does notheat up the heat sink so fast, so that less energy is necessary forattaching the chip to the heat conductive layer by softening theundercoating.

In a preferred variant of the method of the invention, the methodadditionally comprises a further method step before step F), forming acontact point on the heat conductive layer at a distance from theundercoating as step H), wherein the energy in step F) is applied to theheat conductive layer over the contact point.

The contact points should be formed in a distance which is large enoughthat the undercoating and the chip are not being damaged when the energyis applied to the contact point. The contact point is formed in a waythat the absorbance of energy is increased within this contact pointcompared to the surrounding heat conductive layer. When the energy isapplied to the contact points, it is absorbed by the contact pointstransfer through the heat conductive layer to the undercoating so theundercoating could be softened without applying energy directly to theundercoating.

In a variant of the method, a laser is used in step F) for applying theenergy.

The contact points can comprise an absorption material which can be apigmental material for example. Hereby, the absorption maximum of theabsorption material can be matched to the laser wavelength. Due to thata high quantity of absorption can be reached without distributing anabsorption material over the whole surface of the heat conductive layer.The absorption material can be located only on the surface of thecontact point, for example by printing it onto the surface. But it canalso be distributed through the whole contact point, for example bymixing the absorption material with a material the contact point is madeof. For example carbon black could be used as pigment material.

In another variant of the method, a soldering rod is used in step F) forapplying the energy.

In this case even solder temperatures could be applied to the contactpoints which would destroy the chip if these temperatures would beapplied directly to the undercoating. Because the energy which isapplied to the contact points of the surface of the heat conductivelayer in a distance to the chip is distributed through the whole heatconductive layer before it reaches the undercoating. The solder processcan be, for example, hotbar soldering, robotic tip soldering or handsoldering. With a large hot bar many points in a line could be selectivesoldered at once. With robotic tip soldering one or more points touchinga board could be soldered a one time.

In another variant of the method in step H), a plurality of contactpoints is formed and the energy in step F) is simultaneously applied toall contact points.

If the light emitting device comprises more than one contact point theenergy applied to these contact points should be applied to thesecontact points at the same time. In this variant, a higher uniformity ofheat distribution within the heat conductive layer and therefore ahigher uniformity of softening the undercoating can be reached. A highuniformity of heat distribution during the softening of the undercoatingis important to avoid a shift of the chip, which is located on theundercoating. Furthermore, it is important to manufacture a lightemitting device having a uniform thickness of the undercoating beneaththe light emitting chip.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section of one embodiment of the lightemitting device,

FIG. 2 shows a schematic cross-section of another embodiment of thelight emitting device,

FIG. 3 shows a schematic cross-section of another embodiment of thelight emitting device including a casing surrounding the light emittingchip,

FIG. 4 shows a schematic cross-section of another embodiment of thelight emitting device,

FIG. 5 shows schematically a stack modular system comprising a pluralityof the light emitting devices of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the cross-section of a light emitting device.The light emitting device comprises a heat sink 5 with an enlargedsurface area on the underside of the device where the heat can betransferred to the surrounding area. The heat sink 5 is made out ofmaterial with good heat conductivity for example a metal like aluminiumor copper. On the heat sink 5 a dielectric layer 4 is arranged. Thisdielectric layer 4 electrically insulates the heat sink 5 from thelayers which are arranged on the dielectric layer 4 opposite to the heatsink 5. The dielectric layer 4 could be made of an anorganic materiallike a metal oxide or of a polymeric organic material like a plastic,for example T-Iam SS 1KA04®.

The dielectric layer 4 is furthermore thermo conductive in order totransfer heat from the layers, which are located on the dielectric layer4 to the heat sink 5. The heat conductive layer 2 is arranged on thedielectric layer 4. The heat conductive layer 2 comprises material withgood heat conductivity, for example copper or graphite. In the middle ofthe heat conductive layer 2, the light emitting chip 1 is located, whichis attached to the heat conductive layer 2 by means of the undercoating6. The chip 1 is attached to the heat conductive layer 2 by theundercoating in a mechanical and heat conducting way. It is possiblethat the chip is also attached to the undercoating 6 in an electricallyconducting way. Also the heat conductive layer can be a part of acircuit. In a distance to the undercoating 6, there are two contactpoints 3 located in the heat conductive layer 2, both contact pointshaving the same distance to the undercoating 6. These contact points 3makes it possible to apply heat to the device, which is then distributedthrough the whole heat conductive layer 2. The contact points 3 are alsomade of a high thermal conductive material like copper or graphite. Thechip 1 is electrically connected through the heat conductive layer 2.The heat conductive layer 2 can be part of a circuit and can be made ofcopper.

FIG. 2 shows a cross-section of another embodiment of the light emittingdevice. This embodiment comprises, additionally to the one which isshown in FIG. 1, a circuit 7. This circuit 7 is located on the backsideof the light emitting device and can be, for example, a driver circuit.The circuit 7 is located on the opposite side of the heat sink 5 wherethe chip 1 is located. The circuit 7 is attached to another heatconductive layer 2 which also comprises two contact points 3. The secondheat conductive layer 2 is also attached to a dielectric layer 4. Thecircuit 7 can be electrically contacted with the chip 1 (not shown inthe figure). The chip 1 could be connected to the circuit 7 via a holeor via a rivet through the heat sink 5.

The advantage of locating the circuit 7 on the other side of the heatsink 5 is that the chip 1 and the circuit 7 are not interfering witheach other in a visual, aesthetic or thermal way. Also, the chip 1 andthe circuit 7 could be attached to the device in the production processin a way without influencing the other device by applying heat to thesurface. The heat of the circuit 7 can also be transported through thehigh conductive layer 2 to the heat sink 5. The device comprises a firstcircuit layer 16 in the heat conductive layer 2 and a second circuitlayer 17. The circuit layers 16/17 are mad of an electrically conductivematerial, for example copper. The circuit layers 16/17 could be used forthe electrically contact for the chip 1 and the circuit 7. There is anelectric pass-through 13 which connects the first circuit 16 with thesecond circuit layer 17 and the two heat conductive layers 2. Therebythere is an electrically connection between the chip 1 and the circuit7. Instead of the electric pass-through 13 the electrical connectioncould go around the heat sink 5 with a wire for example. Anotheralternative to the electric pass-through 13 is an isolated wire throughthe heat sink 5. There are also embodiments possible, where the circuit7 is on the same side of the heat sink 5 as the chip 1.

FIG. 3 depicts a cross-section of another embodiment of the lightemitting device. This embodiment further comprises, compared to theembodiment which is shown in FIG. 1, a casing 8 including an opticelement 9, which could be a lens or a disperse-plate for example. Thecasing 8 could be made of a material like a gel, which can be indexmatched to the optic element 9. The material is chosen in a way, thatthe refraction index of the casing 8 is similar to this of the opticelement 9, to reduce the reflection at the interface of both materials.In another embodiment, the casing 8 can also be only a frame whichcarries the optic 9 so that between the chip 1 and the optic 9 there isonly air. The optic element 9 can be used to manipulate the radiationemitted from the chip 1.

FIG. 4 shows a cross-section of a further embodiment of the lightemitting device in form of an electric bulb. The bulb comprises a heatsink 5 which forms the lower part of the body; the upper part of thebody is formed by a curved lens 11. In the middle of the bulb, the chip1 is located covered by an optic element 12. The chip 1 is arranged onthe heat conductive layer 2, also connected to the circuit 7, the heatconductive layer 2 is arranged on the dielectric layer 4 whichelectrical isolates the heat conductive layer 2 from the heat sink 5. Atthe lower end of the heat sink 5 there is a circuit 7 which iselectrically contacted to the chip 1 through an electric pass through13. The device also comprises a bulb connection 10 on the bottom whichcan be connected to the circuit 7.

FIG. 5 schematically shows a cross-section of a stackable modularsystem. In FIG. 5 are three modules 14 shown, which are stacked on topof each other and are connected via a connection part 15. The modules 14can be electrical connected via the connection part 15. Each stackablemodule 14 comprises a plurality of chips 1. These chips 1 could be chips1 like the one shown in FIG. 1 for example. Hereby, the body of thestackable modules 14 can be the heat sink 5 or an additional casing.Each of the chips 1 can be located on a single separate heat conductivelayer 2 or each stackable module 14 can have one large heat conductivelayer 2. On the backside of the stackable module 14, there could belocated a circuit 7 which can control all chips 1 of one module 14. Thechips can emit the same or different coloured lights such as red, green,blue, white or pink. In the case of light emitting device with multipleemitters, a combination of coloured emitters can be arranged in onemodule in order to create tuneable colours or add infrared content tothe spectrum of the light engine. Separate chips 1 in the module 14could be addressed separately to control the light output of individualchips 1.

Spectrum converters such as phosphors could be applied over the chip 1or the optic elements 9 or 12. In addition the converter material alsocould be included in the material of the casing 8, the lens 11 or theoptic elements 9 or 12.

Optic elements 9/12 may be placed above the chips 1 to change theradiation characteristics of the chip 1 and/or to increase the opticalout coupling. These optic elements could be arranged over the chip 1with or without an air gap between the respective chips 1 and the opticelements.

Light emitting devices of this invention could be designed to stacktogether in a luminary design. The device could comprise modules in formof hexagons and could form a “honeycomb”.

The above description of the invention using the exemplary embodimentsis not to be understood to mean a restriction of the invention thereto.Rather, the inventive concept set out in claims 1 and 15 can be appliedfor a large number of very different designs. In particular, theinvention also covers all combinations of the features cited in theexemplary embodiments and in the rest of the description, even if thesecombinations are not the subject matter of a patent claim.

1. A light emitting device comprising: a heat sink; a dielectric layerarranged on the heat sink; a heat conductive layer arranged on thedielectric layer; an undercoating arranged on at least a part of theheat conductive layer; a light emitting chip attached to the heatconductive layer by the undercoating; and an optical element arranged ina beam path of the light emitting chip, wherein the light emitting chipis electrically conductively connected to at least a part of the heatconductive layer.
 2. The light emitting device according to claim 1,wherein the heat conductive layer comprises a contact point in adistance to the undercoating.
 3. The light emitting device according toclaim 1, wherein the thermal conductivity of the heat conductive layeris greater than 350 W/m*K.
 4. The light emitting device according toclaim 1, wherein the thermal conductivity of the heat conductive layeris greater than the thermal conductivity of the dielectric layer.
 5. Thelight emitting device according to claim 2, wherein the contact pointhas a geometric form comprising tines, waves or groovings such that asurface of the contact point is enlarged compared to a surface of theheat conductive layer and the absorption of thermal energy is increased.6. The light emitting device according to claim 2, wherein the contactpoint comprises an absorption material that increases the absorption oflight.
 7. The light emitting device according to claim 2, wherein thecontact point and the heat conductive layer contain the same heatconductive material.
 8. The light emitting device according to claim 2,comprising a plurality of contact points.
 9. The light emitting deviceaccording to claim 1, wherein the undercoating comprises a soldermaterial.
 10. The light emitting device according to claim 1, whereinthe melting point of the undercoating is lower than the melting point ofthe conductive layer.
 11. The light emitting device according to claim1, wherein a circuit is arranged on a side of the heat sink which isfacing away from the light emitting chip.
 12. A light emitting devicecomprising: a heat sink; a dielectric layer arranged on the heat sink; aheat conductive layer arranged on the dielectric layer; an undercoatingarranged on at least a part of the heat conductive layer; a lightemitting chip attached to the heat conductive layer by the undercoating,wherein the light emitting chip is electrically conductively connectedto at least a part of the heat conductive layer; a circuit arranged on aside of the heat sink which is facing away from the light emitting chip;and an electric pass-through that electrically conductively connects atleast one of the light emitting chip and the heat conductive layer tothe circuit.
 13. A light emitting device comprising: a heat sink; adielectric layer arranged on the heat sink; a heat conductive layerarranged on the dielectric layer; an undercoating arranged on at least apart of the heat conductive layer; a light emitting chip attached to theheat conductive layer by the undercoating; and an optical elementarranged in a beam path of the light emitting chip, wherein a circuit isarranged on a side of the heat sink which is facing away from the lightemitting chip.
 14. The light emitting device according to claim 13,wherein at least one of the light emitting chip and the heat conductivelayer is electrically conductively connected to the circuit by anelectric pass-through.